In vertebrates lactate dehydrogenase (LDH) exists as a tertrameric enzyme having identical subunits with a molecular weight of 36,000; it is specific for L-lactate. On the other hand, an investigation of LDH's in invertebrates and microorganisms reveals a far greater diversity in specificity, subunit structure, and allosteric properties. With both the primary structure and crystal structure of dogfish M4 and pig H4 LDH known, it becomes meaningful to compare structural homologies which may exist in some of these other LDH's. LDH provides an excellent model for investigating the evolution of enzyme specificity, subunit interaction, and allosteric control and for correlating significant differences in enzyme properties with changes in primary structure. LDH from lobster tail shows unusual sigmoidal kinetics which can be correlated in part to the aggregation state of the enzyme in which the dimer and tetramer are in equilibrium. We have devised a technique for rapidly and selectively isolating peptides from the active site regions of the molecule. The unique nature of lobster tail LDH in comparison to vertebrate LDH's is indeed reflected in a sequence comparison of these active site regions. The complete primary structure is now being elucidated. Again in contrast to vertebrate LDH's, other LDH's exist which are specific for D-lactate. These enzymes are found in mollusks, arachnids, and in several bacteria and are consistently dimeric having a molecular weight of 72,000. We have purified one of these LDH's from horseshoe crab and are presently isolating peptides from the active site region in an effort to correlate the known changes in enzyme specificity with differences in amino acid sequence. A final LDH under investigation has been isolated from Lactobacillus casei. This enzyme is tetrameric although at high pH readily dissociates to a monomer. It is unique in that it is totally dependent on fructose-1, 6-diphosphate for enzymatic activity. Structural studies with this enzyme are in progress. BIBLIOGRAPHIC REFERENCE: Taylor, S.S., Amino Acid Sequence of Dogfish Muscle Lactate Dehydrogenase, J. Biol. Chem. (1977) 252, in press.